Electrochemical conversion cells, commonly referred to as fuel cells, produce electrical energy by processing reactants, for example, through the oxidation and reduction of hydrogen and oxygen. A typical polymer electrolyte fuel cell comprises a polymer membrane (e.g., a proton exchange membrane (PEM)) with catalyst layers on both sides. The catalyst coated PEM is positioned between a pair of gas diffusion media layers (DM), and a cathode plate and an anode plate (or bipolar plates (BPP)) are placed outside the gas diffusion media layers. The components are compressed to form the fuel cell.
Fuel cell stack operation at very high current density (for example, about 2 A/cm2), high electrical contact resistance between the bipolar plate and the gas diffusion media or between the gas diffusion media and the electrodes generally results in unacceptably large ohmic losses (electrical resistance). Although fuel cells rarely operate at higher current density (for example, greater than about 1 to about 1.5 A/cm2), it is necessary to be able to achieve these higher power generation capabilities with minimal ohmic losses. The voltage loss in each cell due to ohmic resistance increases with current density (ohmic voltage loss=total ohmic resistance×current density). Consequently, the higher the current density, the greater the benefit of reducing interfacial resistance.
What constitutes unacceptably high ohmic losses in a particular application depends on the cost trade-off between ohmic loss and compression level. The level of ohmic loss in each cell determines the maximum current density achievable, and the associated active area required, to produce a desired amount of power at a given voltage and total number of cells. The higher the ohmic losses are, the lower the maximum current density can be for a given cell voltage, and thus the larger the required cell active area must be to achieve a desired power. On the other hand, there is an additional cost associated with increasing stack compression to achieve sufficiently low ohmic loss (for a given cell active area).
Currently, low electrical resistance at the interface between a steel plate and the diffusion media can be achieved by using a highly conductive, thin coating on the plate. The coating materials can include gold, carbon graphite, or other electrically conductive materials that can be quickly deposited in a controlled, nanometer-scale thickness and are compatible in the fuel cell operating environment. The use of conductive adhesives, such as silver-filled epoxy and solders, has also been tried to reduce electrical resistance and mechanically bond the diffusion media to the plates.
One problem with these approaches has been the cost. Also, the adhesives are susceptible to degradation over time, particularly in humid environments, such as a fuel cell, resulting in increased resistance over time.
Therefore, there is a need for improved diffusion media, coatings, or materials at the interface between the diffusion media and plate, as well as for methods of making them.